In a conventional tank-based aquaculture operation, adequate water is furnished to each tank so that dissolved oxygen is maintained at appropriate levels and both dissolved and solid wastes are kept at acceptable concentration by dilution with the continual input of clean water. The de-oxygenated water with the dissolved and solid wastes are continually swept out of a drain system which is centrally located in the tank. Since the solids are automatically swept to the centre-bottom of a standard circular culture tank, the exit or drain for solid wastes and waste water to be filtered is located at the centre-bottom of the tank.
Once the solids and waste water exit the tank and enter a small diameter effluent collection pipe, the turbulent high velocity water flow causes the solid waste to be broken down into smaller particles with the resultant creation of fine particulates.
If the fish culture water is to be subsequently re-used for further fish culture, solid and dissolved wastes must be removed and oxygen supplemented. If the waste water is to be returned to a surface water source, (lake, river or the like), current environmental regulations usually require at least removal of solid wastes before it can be returned. This is traditionally accomplished by either using a settling lagoon or expensive mechanical filtration devices. The fine particulates in the waste water require a considerable time to settle out in the lagoon or require complex technology for mechanical removal since the fines require frequent back flushing to prevent filter clogging.
Because of the large flows in even modest sized fish farm operations the settling lagoons (the most common approach) have to be very large to provide adequate time to settle out the fine suspended particulates. The lagoons are generally equal in surface area to the area occupied by the tank farm itself. Because of the size of the lagoons, they are generally only cleaned once or twice per year. The solid wastes that settle to the bottom of the lagoon decompose and generate dissolved wastes which are carried out of the lagoon with the clarified effluent water and therefore enter the receiving surface water. Significant quantities of both nitrogen and phosphorus are released which are the major nutrients responsible for eutrophication of surface waters.
An adequate water filtration system for aquaculture requires adequate fine particle filtration. Biological filters in general work most efficiently when the water, which is to be treated, is prefiltered to minimize suspended solids which otherwise can coat and smother the aerobic bacterial colony and promote the growth of heterotrophic bacteria. When used in aquaculture applications, a mechanical particle filter can be used to remove the larger particulate waste such as fecal pellets and food pellets. However, it is desired to provide a treatment system which effectively removes solid wastes without removing significant amounts of water.
A fluidized bed configuration for filtering dissolved waste water, as shown in my U.S. Pat. No. 5,055,186 issued Oct. 8, 1991, (VanToever), was designed strictly to optimize conditions for bacterial culture using floating low density plastic media in an aerated, fluidized bed.
However, a system which includes a more static bed configuration, thereby reducing the energy requirements otherwise required for a fluidized bed configuration, is desirable. It is also desirable to separate out and remove solid wastes with as little associated water loss as possible and with a minimum use of potentially clogging screens.
It would be desirable in any new treatment system to incorporate a solid waste separation system associated with each tank wherein solid wastes, inherently swept to the centre-bottom of a conventional circular fish tank by the circular flow pattern in the tank, can be selectively removed via a separate exit with a very small flow of water. This can be achieved when a second, separate exit from a tank is provided for clear waste water flow. Clear waste water--clarified water--removal in my new design is via a central stand-pipe with the opening located off the floor, e.g. about mid-water height in the fish tank. The clear water drain is located up off the bottom since if it is located too close to the tank floor, solids would be resuspended from the bottom by the high velocity clear water flow being removed.
Further, early experiments conducted with static beds of low density media revealed that fine particles were mechanically filtered out when waste water was passed through the bed in a fashion similar to a sand filter. Experiments using an upflow configuration wherein the media was retained in the filter by a top screen experienced fine particle accumulation in the filter bed. With the increased pressure caused by restricted water flow, the media were forced against the top screen whereby the bed was compacted and water flow decreased. This necessitated periodic fluidization of the bed with air and the rinsing of particles from the media bed and resulted in a considerable amount of backflush water to rinse out the wastes.
U.S. Pat. No. 4,454,038 to Shimodaire et al granted Jun. 12, 1984 discloses apparatus and process for biological treatment of waste water in a downflow operation. The apparatus includes a reactor (filter tank) having a feeding pipe and a distribution pipe at the top thereof for introducing waste water into the reactor and a withdrawing pipe at the bottom thereof for removing treated water. A particulate carrier or media, has a specific gravity less than that of water whereby the media provides a substantially fluidized bed in a downflow, waste water to be treated. The specific gravity of the media is preferably less than 0.9. In the Shimodaire et al apparatus, water is trickled all over the surface of the floating bed of media and a central draft tube in the aerobic filter design is provided, whereby air is diffused under pressure to carry media, which have come down to the bottom of the reactor by excess attachment thereto of microbes, are caused to rise in the draft tube whereby excess microbes are sloughed off while the media rises up through the draft tube due to the air bubbles. Without a draft tube, a physical stirrer is required to impart a shearing force to the fluidized bed. Further, it appears media material which is caught in the draft tube must also be distributed to the upper surface of the bed through the distribution means. There is no indication of how solid wastes are removed from the reactor and it is clear that screening of the clarified effluent would be required.
It has also been considered necessary in prior art downflow filters to have an elongate, narrow reactor in order to achieve the significant flow rates required to fluidize the bed, thereby making the use of same difficult in areas with height limitations.
Accordingly, it is desirable to provide a downflow biofilter system wherein the filter tank may be relatively wide relative to its height with less violent fluidization and more of a gentle agitated static bed configuration which combines an effective biofilter with a fine particle filter separation and without the need to backflush or periodically fluidize the bed to rinse out waste.
In designing an effective biofilter, it is desirable to provide a thin (1-2 micron) layer of biofilm to ensure that nutrients and oxygen (in aerobic systems) can be transported to the micro-organisms in the interior of the film. If the film becomes too thick, the micro-organisms in the interior will smother, die and decay, which leads to anaerobic conditions, an ineffective filter.
It is further desirable that the filter media be designed to provide a very high surface area per unit volume to maximize the area for growth of micro-organisms (biofilm). At the same time the filter media must continually shed excess biofilm as it grows and accumulates in order to prevent the individual media pellets from adhering together and clumping into masses. This is a problem with conventional biofilters wherein the accumulating biofilm will gradually plug the interstitial spaces between whatever media type is used which causes channeling of the filtrate and smothering of the active biofilm.